Apparatus for indicating errors in inclination for inertial navigation

ABSTRACT

An inertial navigation and guidance system includes a gyroscope and accelerometer mounted on a platform for establishing a common normally horizontal plane. The gyroscope is precessed at a constant rate and a pulse signal is produced whose frequency varies in accordance with the deviation of the platform from the horizontal. An electronic gate compares the pulse signal with a constant frequency reference signal. A further pulse signal is produced in response to the accelerational forces acting on the accelerometer in the common plane. This latter pulse signal is applied together with the output of the gate and with a constant frequency reference signal to an adder which provides an output signal related to the acceleration of the system.

United States Patent Granqvist [451 June 27, 1972 APPARATUS FORINDICATING [56] References Cited ERRORS IN INCLINATION FOR UNITED STATESPATENTS INERTIAL NAVIGATION 2,396,617 3/1946 Von Den Steinen ..73/5042,856,772 10/1958 Strihafka ..73/504 [72] 2,942,864 6/1960 Sikora..73/504 [73] Assignee: AGA Aktlebollg, Lidingo, Sweden PrimaryExaminer-James J. Gill Flledl J y 13, 19.67 AssistantExaminer-l-lerbertGoldstein i pp 653 075 Attorney-Larson andTaylor Related 0.8.Application pm [571 ABSTRACT [63] Continuation-impart of Ser.No.-467,l82, June I], An inertial navigation and guidance systemincludes a 1965 abandoned which is a continuatiomimpan of gyroscope andaccelerometer mounted on a platform for s 169 538 29'1962 abandonedestablishing a common normally horizontal plane. The gyroscope isprecessed at a constant rate and a pulse signal is vproduced whosefrequency varies in accordance with the [30] Foreign Appliclu-on Prlomydeviation of the platform from the horizontal. An electronic Feb. 6,1961 Sweden ..l200/6l 8 compares the Pulse Signal with a mnstamfrequency a reference signal. A further pulse signal is produced in [52]US Cl 73/504 33/226 response to the accelerational forces acting on theac- 51 I .Cl G01, 15/14 celerometer in the common plane. This lam" pulseSignal is l 1 I226 applied together with the output of the gate and witha con- [58] held f stant frequency reference signal to an adder whichprovides an output signal related to the acceleration of the system.

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INVENI'OR CARL-ERIK GRANQVIST BY Jw w 34 /92 ATTORNEYS APPARATUS FORINDICATING ERRORS IN INCLINATION FOR INERTIAL NAVIGATION CrossReferences to Related Applications FIELD OF THE INVENTION This inventionrelates to an inertial navigation or guidance system comprising ahorizontalaccelerometer platform.

BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART Inertialnavigation and guidance systems primarily rely on devices for sensinghorizontal accelerations from which velocity and distance can beobtained by successive integration. Such devices or accelerometers, mustbe mounted on platforms that are carefully maintained in a horizontalplane in order to prevent the accelerometer from being affected bygravitational acceleration.

In copending United States application Ser. No. 86,975, filed Feb. 3,I961, now U. S. Pat. No. 3,304,788 granted on Feb. 21, 1967, wehavealready described a free gyroscope which can be made to precess at acontrollable rate and in which a change in the .controlled rate ofprecession can be measured by a scanning device which delivers a signalin the form of a low frequency signal that directly represents anydeviation from said controlled rate of precession. If the rate ofprecession is controlled by reference to the speed and direction oftravel of a vehicle, derived for instance from the indications of anaccelerometer, it can be controlled to remain constant with reference tothe horizontal. The principle of keeping an accelerometer platformstable by driving it from a stable platform by reference to theintegrated signals of an accelerometer is well known to the art. The useof a precessing gyroscope in which the rate of precession is thuscontrolled removes the need of a stable platform, since the signalsdelivered by the scanning device can be directly resolved in asine-cosine resolver and the outputs of said sine-cosine resolver usedto activate torquers for tilting the accelerometer platform until theerror signal disappears. Thus the gyroscope which is itself mounted onthe accelerometer platform will directly operate to keep this platformin the required horizontal position.

Nevertheless,if an accelerometer is mounted on such a platform, whichresponds to accelerations in the plane of the platform, transient errorsin the signals derived from the accelerometer will still occur duringthe continuous process of reerection, and in course of time these willfalsify the results of the integration.

BRIEF DESCRIPTION OF THE INVENTION It is therefore the object of thepresent invention to provide an apparatus which will automaticallyoperate to eliminate this source of error.

It is a further object of this invention to provide a novel and improvedinertial navigation and guidance system.

To this end the present invention discloses an inertial navigation andguidance system comprising at least one gyroscope and one accelerometer,a platform means for establishing a common normally horizontal plane inwhich both the gyroscope and the accelerometer are mounted, a gimbalmounting for said gyroscope comprising inner and outer gimbals mountedon an inner and an outer gimbal shah, respectively, means for processingthe gyroscope at a constant rate with reference to the horizontal aboutthe outer gimbal shaft, a first indicating means connected with theprecessing gyroscope for generating a low frequency pulse signal, thephase position of which varies when the platform means deviates from thehorizontal, an electronic gate having two inputs and one output andmeans for supplying said low frequency pulse signal to one input andmeans for supplying a constant frequency reference signal to the otherinput, an adder with at least two inputs, one of which is connected withthe output of the electronic gate, a second indicating means connectedwith the accelerometer for generating a pulse signal in response toaccelerational forces acting on the accelerometer in the said commonplane, and means for supplying said accelerometer pulse signal toanother input of the adder.

Other objects and advantages of the present invention will be apparentfrom the accompanying description when taken in conjunction with thefollowing drawings, wherein:

FIG. I is an overall perspective view of the platfonn which carries anaccelerometer and a gyroscope together with the gimbal arrangementcarrying the plate and its servo motor; and

FIG. 2 is a block wiring diagram showing the relationship of the severalcomponents of a specific embodiment of the present invention;

FIG. 3 illustrates the details of the accelerometer of FIGS. 1 and 2;

FIG. 4 illustrates the details of the gyroscope of FIGS. 1 and FIG. 5 isa detail representation of the connections of the resolver of FIG. 1;

Referring to FIG. 1, the horizontal platform, indicated generally by thenumeral 10, carries an accelerometer 20 and a gyroscope 21 constructedin accordance with FIGS. 2 4. It is pointed out that the platform neednot necessarily exist as a physical structure as such, but it issufficient that a plane can be derived which takes the position whichthe platform should have.

The platform is supported by a shaft 1 I mounted in an inner gimbal 12which in turn is pivotably suspended by a shaft 13 in an outer gimbal l4pivoting on a shaft 15 supported in bearings 16 in the frame of thevehicle containing the inertial guidance or navigation system. Asine-cosine resolver 17 is associated with the shaft 11 supporting theplatform 10 in such a manner that the movable coils 54 of the resolver17 are mechanically connected with the shaft 11 by means of a disc 86(see FIG. 5), while the fixed coils of this resolver are connected withthe inner gimbal 12 by means of the housing 87 of the resolver 17 and abar 88 (FIG. 5). Moreover, a first servo motor 18 is included in theshaft 13 suspending the inner gimbal 12 in the outer gimbal 14. Asimilar servo motor 19 is provided in the shaft 15 which carries theouter gimbal 14 in the bearings 16. The rotors of servo motors, I8 and19 are connected with the shafts l3 and 15, respectively, while thestators of these servo motors are connected with the gimbal l4 and oneof the bearings 16, respectively. The purpose of these servo motors isto apply torque to the gimbals to maintain the pladorm permanently in anexact horizontal position.

To permit the actual position of the platform to be determined withsufficient accuracy, means must be provided for indicating everydeviation with an extremely high degree of precision. For this reason itis desirable also to provide a gyroscope on the horizontal platform byreference to which the position in space of the horizontal platform canbe monitored and continuously corrected. The gyroscope may beconstructed in accordance with FIG. 6 of the above mentioned US. Pat.No. 3,304,788. The manner in which this is carried into effect in thepresent invention is shown in FIGS. 2 and 4.

In this diagram the horizontal accelerometer platform itself is notshown. The accelerometer mounted on the platform is generally indicatedat 20, whereas a gyroscope similarly mounted on the platform isgenerally indicated at 21. The system includes circuitry employing avariety of electronic elements of a kind well known to the art. Theindividual elements are merely shown in block form because theirdetailed construction requires no particular description and has nodirect bearing upon the invention.

The accelerometer 20 in the illustrated embodiment is presumed to be ofa kind that has been described in greater detail in our copenclingUnited States application Ser. No. 91,728, filed Feb. 27, 1961 now U.S.Pat. No. 3,304,786, granted on Feb. 21, 1967, and vital elements thereofare shown in FIG. 3. Briefly, this accelerometer comprises at least twoweights 71, 72 (FIG. 1) eccentrically suspended between springs 73 on arevolving disc 74 (FIG. 3). As long as the revolving disc is horizontaland the vehicle carrying the system is not itself subjected tohorizontal accelerations, the only accelerations acting on the weightsare the accelerations of the centrifugal forces which displace theweights against the resistance of the springs. The radial position ofthe weight 71 is sensed by pick-off 75 and the radial position of weight72 (FIG. 1) is sensed by similar pick-off 75 (FIG. 2) which are suppliedwith a carrier frequency over a line 27 and which modulate the carrierfrequency constant in dependence upon the magnitude of radialdisplacement of the weights, i.e., upon the speed of rotation of therotating disc. The weights of the accelerometer are suspended inquadrature and the outputs of the two pick-offs are taken to asine-cosine resolver comprising input coils 22 and 22 connected with androtating with the pick-offs 75 and 75'. The sine-cosine resolver alsocomprises two mixed output coils 23 and 32 in quadrature and each one ofthese output coils cooperates'with both input coils 22 and 22. As aresult thereof the amplitude and the duration of a halfperiod of themodulated output voltage of either of the fixed output coils 23 and 32is indicative of the component of the acceleration in the directionrepresented by said fixed output coils as described in our said U.S.Pat. No. 3,304,786. If the sine-cosine resolver is arranged to resolvethe modulating frequency into its north-south and east-west components,for instance by slaving the platform to a north-seeking device (notshown inthe drawing) so that the output coils 23 and 32 are oriented inthe directions north-south and east-west, the output of said coils 23and 32 will directly indicate the vector components of this linearacceleration in terrestrial coordinates. Since each of these outputcomponents is further processed in a similar system of apparatus, thefollowing description may, .with convenience, be confined to describingonlyone of them, namely the component derived from output coil 23.

The carrier frequency supplied to the accelerometer is derived from anoscillator 25 which is controlled by a crystal 26. Since the constancyof the frequency of a crystal controlled oscillator is the better thehigher its frequency, it is preferred that the oscillator frequencyshould be a high multiple of the carrier frequency supplied to thepick-ups of the accelerometer via line 27. Consequently the oscillatorfrequency is first taken to a frequency divider 29. Moreover, since inthe illustrated system, as will later be described, it is also desiredto supply a a frequency as low as three cycles per second, a

convenient frequency for the oscillator would be say 98,304

cycles per sec. The frequency divider 29 may then be arranged to stepdown this frequency in the proportion 1:25 or 1:32, so that thefrequency appearing in lines 27 and 28 after amplification in anamplifier 30 will be 3,072 cps. The motor for driving the accelerometerat a constant speed may also be controlled by a frequency derived fromthe oscillator although this is not specially shown in the drawing.

The phase of the modulating frequency n the output from coil 23 is nowcompared in a device 24 with the phase of the oscillator frequency as areference in line 28. As is described in detail in said U. S. Pat. No.3,304,786 for corresponding elements, the output in coil 23 will includea low frequency envelope voltage, the two half-waves of which are ofequal duration if the accelerometer is not subjected to anyacceleration. If an acceleration is measured, the two half-waves are ofdifferent duration, i.e., the voltage envelope will be distorted suchthat one half-wave extends over more than half of the period ofoscillation. The output of coil 23 is, therefore, rectified, amplifiedand limited by device 24 to produce a series of voltages which arecompared in device 24 to a reference from oscillator 25. If the phasedisplacement of the modulating frequency from coil 23 varies,conformably varying pulses of carrier frequency will appear in theoutput of the device 24. The number of these pulses will then correspondto the magnitude of the component of acceleration in one coordinatedirection. These pulses are amplified in an amplifier 31 and thensupplied to one input circuit of an adder 33. The adder device at 33 isat the same time supplied with pulses derived from the frequency inlines 27 and 28 after this has been treated in the same way at theoutput from device 24. The corresponding electronic devices are notspecially shown in the drawing in order to avoid complications and arepresumed to be contained in line 34. Line 27 supplies the relativelyhigh frequency alternating current to pick-offs 75 and 75'. The deviceat 33 adds the two sets of pulses to provide an output at 35 which istaken to further electronic means which also receive the output from asimilar system associated with the second coil 32 of the sine-cosinedivider, and which are devised to provide a digital indication of thedirection and magnitude of the acceleration of the vehicle. Furtherdetails of such a system are described in said U.S. Pat. No. 3,304,786and as such they form no part of the present invention.

The accuracy of the indications derived from the accelerometerdecisively depends upon the rotating disc or platform 10 beingmaintained exactly in the horizontal. The accelerometer platform is keptin the required horizontal position by gyroscopic means. In a normalsystem two gyroscopes will be used, but for the sake of convenience thedescription will first be confined to the nature and functions of onlyone gyroscope generally indicated at 21.

The aforesaid 3,072 cps. output of the frequency divider 29 is taken toa further frequency divider 38 which further reduces this frequency inthe proportion of 1.2 or 1.8, so that the frequency appearing in line 39will be 384 cps. Part of this frequency is applied to yet anotherdivider 40 which again reduces the same in the proportion of 1.2 or1.128 to supply a frequency of only 3 cps. via line 42, on the one hand,to a phase detector 43 and on the other hand, to an electronic gate 37via line 41.

Gyroscope 21 is of the type described, for example, in U.S. Pat. No.3,304,788 referred to above and the basic construction thereof is shownin FIG. 4. Gyroscope 21 is one of the gyroscopes included in theinertial navigation system proper, and generally, functions to maintainthe accelerometer platform horizontal. Considering the basic operationof gyroscope 21, the rotor speed of the gyro rotor (not shown) in thegyrohousing 77 is precisely controlled by the 384 cps frequency derivedfrom the frequency divider 38, after amplification in amplifying means61, through line 47. The gyroscope 21 is a free gyroscope suspended in agimbal 78 and mounted on the accelerometer platform by means of a shaft79 and bearings 80. By the application of a constant torque by means ofa spring 81, the gyroscope is first made to precess in the outer gimbalbearings 80 at a constant rate in inertial space or at a rate whichremains constant when the vehicle is stationary.

For the compensation of drift the outer gimbal carries a pick-off 82(see FIG. 4) which is inserted between the inner gimbal shaft 83 and thegimbal 78, for instance in such a way that a moveable armature isconnected with the shaft 83 and an E-core (not shown) cooperating withthis armature is connected with the gimbal 78. A corresponding elementhas been shown in said U. S. Pat. No. 3,304,788.

The pick-ofi arrangement 82 senses any tendency of the inner gimbal tomove in relation to the outer gimbal, the output from the pick-ofi' 82being used after amplification in an amplifier 84 to control a servomotor 48 which applies a compensating torque to the suspensionconteracting any such tendency. The frequency supplied to this pick-offis likewise that delivered by amplifier 61 through line 47 (see alsoFIG. 2).

Associated with a disc 44 mounted on the gimbal shaft 79 is a scanner,for instance in the form of a light source 45 and a photocell 46, asshown in FIG. 2, in which the opaque and transparent segments of therotating disc induce current pulses at a frequency depending primarilyupon the speed of precession of the gyroscope. The scanner is mounted onthe accelerometer platform and the rate of precession of the gyroscopeis so maintained that the frequency of the pulses delivered by thescanner is exactly 3 cps when the vehicle is at rest and the platformhas been horizontalized. The frequency delivered by the photocell 46 issupplied to phase detector 43 for comparison with the 3 cps. in line 42derived from the oscillator 25. So long as the platform remainshorizontal no signal will appear in the output from phase detector 43.However, as soon as the platform begins to move out of the horizontal,the frequency delivered by the photocell will slightly change and acorresponding signal will appear in the output 51 of phase detector 43.In a modulator 52 this signal modulates the assumed frequency of 384cps. applied through line 53 and the modulated signal is taken to oneinput winding of a resolver generally indicated by 17 (FIGS. 1, 2 and5), having two input windings 54 mechanically connected with the shaft11 as indicated by the dotted line 85 (FIG. 2) and having two outputwindings 55 and 56. The second gyroscope mention ed hereinabove is usedto provide an output signal which is fed in a corresponding way to theother input winding 54 of resolver 17. The resolver resolves the signalssupplied to the two input windings 54 into their sine and cosinecomponents which appear in the coils 55 and 56. These coils areconnected by lines 57 and 58 to the two servo motors l8 and 19, asindicated by the dotted lines in FIG. 1. The servo motors correct theposition of the platform, and when this correction has been made, thesignal in lines 57 and 58 will cease.

From the instant the accelerometer platform deviates from its prescribedhorizontal position to the instant of its restoration, the accelerometerwill naturally produce a signal which includes an unwanted component ofgravity and the output signal in line 35 therefore requires suitablecorrection during this period. According to the invention this isaccomplished by simultaneously feeding the frequency delivered byphotocell 46 to one input of an electronic gate 37 through line 49,whereas the constant 3 cps. frequency derived from the oscillator isapplied through line 41 to the other input of the gate. So long as thetwo frequencies are equal, no pulses appear in the output from gate 37,but as soon as there is a difference in the two frequencies the gatewill pass a number of pulses which are added or subtracted in the adder33 from the number of pulses appearing in its output 35. Since thenumber of pulses appearing in line 36 depends upon the transientdeviation of the platform from the horizontal, the resultant correctionof the number of output pulses in line 35 can be ar' ranged tocompensate the error signal from the accelerometer exactly.Consequently, successive integration of the signals appearing in line 35will provide an exact measure of the ground speed of the vehicle and ofthe distance covered. It will be understood that this invention issusceptible to modification in order to adapt it to different usages andconditions and, accordingly, it is desired to comprehend suchmodifications within this invention as may fall within the scope of theappended claims.

What is claimed is 1. An inertia navigation and guidance systemcomprising at least one gyroscope and one accelerometer, a platformmeans for establishing a common normally horizontal plane in which boththe gyroscope and the accelerometer are mounted, a gimbal mounting forsaid gyroscope comprising inner and outer gimbals mounted on an innerand outer gimbal shah, respectively, means for precessing the gyroscopeat a constant rate with reference to the horizontal about the outergimbal shaft, a first indicating means connected with the precessinggyroscope for generating a low frequency pulse signal, the frequency ofwhich varies when the platform means deviates from the horizontal aboutan axis parallel to the outer gimbal shaft, an electronic gate includingtwo inputs and one output and means for supplying said low frequencypulse signal to one input and means for supplying a constant frequencyreference signal to the other input, said gate producing an outputsignal whose frequency is a function of the frequency difference betweensaid input signals, an adder having a plurality of inputs, one of WhlChis connected with the output of the electronic gate, a second indicatingmeans connected with the accelerometer for generating a pulse signal inresponse to the accelerational forces acting on the accelerometer in thesaid common plane along an axis perpendicular to the outer gimbal shaft,means for supplying said accelerometer pulse signal to a second input ofthe adder and means for supplying a constant frequency reference signalto a third input of the adder, said adder adding the frequencies of theinput signals thereto.

2. A system as claimed in claim 1 and further comprising a first phasedetector with two inputs and at least one output and means for supplyingthe output signal of the accelerometer to one input of the first phasedetector, and means for supplying a carrier frequency to another inputof the said first phase detector, the output of the said first phasedetector being connected to said second input of the said adder.

3. A system as claimed in claim 2 including a second phase detectorhaving first and second inputs and an output, means for supplying areference signal to one input of said second phase detector, means forsupplying said low frequency signal to another input of said secondphase detector, wherein the output signal from said second phasedetector is proportional to the deviation of said platform means from ahorizontal plane about an axis parallel with the outer gimbal shaft, and

correcting means connected to the said output of the second phasedetecting means for restoring the said platform means to the horizontalposition when the platform means has deviated therefrom.

4. A system as claimed in claim 2 wherein said second indicating meanscomprises a pick-off coil for producing a voltage proportional to saidaccelerational forces, a sine-cosine resolver for receiving voltagesfrom said pick-off coil and hav ing an output coil, said output coilbeing connected to one input of said first phase detector.

5. A system as claimed in claim 2 including a pick-off-coil meanspositioned between the gimbals of the gyroscope for sensing relativemovement between the inner and the outer gimbal of the gyroscope.

1. An inertia navigation and guidance system comprising at least onegyroscope and one accelerometer, a platform means for establishing acommon normally horizontal plane in which both the gyroscope and theaccelerometer are mounted, a gimbal mounting for said gyroscopecomprising inner and outer gimbals mounted on an inner and outer gimbalshaft, respectively, means for precessing the gyroscope at a constantrate with reference to the horizontal about the outer gimbal shaft, afirst indicating means connected with the precessing gyroscope forgenerating a low frequency pulse signal, the frequency of which varieswhen the platform means deviates from the horizontal about an axisparallel to the outer gimbal shaft, an electronic gate including twoinputs and one output and means for supplying said low frequency pulsesignal to one input and means for supplying a constant frequencyreference signal to the other input, said gate producing an outputsignal whose frequency is a function of the frequency difference betweensaid input signals, an adder having a plurality of inputs, one of whichis connected with the output of the electronic gate, a second indicatingmeans connected with the accelerometer for generating a pulse signal inresponse to the accelerational forces acting on the accelerometer in thesaid common plane along an axis perpendicular to the outer gimbal shaft,means for supplying said accelerometer pulse signal to a second input ofthe adder and means for supplying a constant frequency reference signalto a third input of the adder, said adder adding the frequencies of theinput signals thereto.
 2. A system as claimed in claim 1 and furthercomprising a first phase detector with two inputs and at least oneoutput and means for supplying the output signal of the accelerometer toone input of the first phase detector, and means for supplying a carrierfrequency to another input of the said first phase detector, the outputof the said first phase detector being connected to said second input ofthe said adder.
 3. A system as claimed in claim 2 including a secondphase detector having first and second inputs and an output, means forsupplying a reference signal to one input of said second phase detector,means for supplying said low frequency signal to another input of saidsecond phase detector, wherein the output signal from said second phasedetector is proportional to the deviation of said platform means from ahorizontal plane about an axis parallel with the outer gimbal shaft, andcorrecting means connected to the said output of the second phasedetecting means for restoring the Said platform means to the horizontalposition when the platform means has deviated therefrom.
 4. A system asclaimed in claim 2 wherein said second indicating means comprises apick-off coil for producing a voltage proportional to saidaccelerational forces, a sine-cosine resolver for receiving voltagesfrom said pick-off coil and having an output coil, said output coilbeing connected to one input of said first phase detector.
 5. A systemas claimed in claim 2 including a pick-off-coil means positioned betweenthe gimbals of the gyroscope for sensing relative movement between theinner and the outer gimbal of the gyroscope.